372 to CORRESPONDENTS* 



Baropieter. MY correfpondent from Edinburgh is reminded, wi4b re^ 



gard to liis projed for a barometer, that no enlargement or 

 diminution of the bore will make the leaft difference in the fcale 

 of the common barometer, confining of a tube or veffel, 

 clofed above, and having its lower end open, and communi- 

 cating with a bafon of mercury of confiderable diameter. 



Subdivifion of The contrivance, received fome time ago from T. I. for 

 an/chain.^ ^^ making an aftronomical infirumenf, in which the angular 

 quantities fliall be meafured by the communication of a chain, 

 ftrapi or firing, poireflTes fo much ingenuity and promife, that 

 it has exercifed the heads and hands of a number of eminent 

 men. Among thefe are Rebert Hooke, for a quadrant ; 

 Mufchenbroeck, for a pyrometer, and many operative men, 

 fuch as SifTon and others, for theodolites and quadrants. — 

 Where the intention of the inftrument is limply to magnify 

 the motion, without any particular attention to precifion, the 

 contrivance has a happy etfe6l ; particularly in public leflures, 

 where a number of fpeclalors may obferve the fame effed at 

 the fame time. It is likewife cheap, and may be carried into 

 efFeft in fituations where the ufe and application of more ac- 

 curate apparatus cannot be referred to. 

 Itcannotbe A flight attention to the fubjeft, will (hew that all contri- 



made very ex- vances of the kind here alluded to muft be confiderably inac- 

 curate. For they demand, 1 , that the wheels flioold be very 

 truly circular : 2, and free from all dirt and impurity : 3. that 

 they be well centered : 4. that the chain or firing (liould be 

 every where of the fame thicknefs : 5, and its tenfion in all 

 pofitions alike, &c. &c. If the quantity of error, taken at 

 a minimum, which mufl arife from thefe and other caufes, be 

 attended to, it will be 4"ound that a fimple divifion of an arc 

 (fubdivided by a fcrew or a nonius) and examined or read ofT 

 by a fmall magnifier, will afford greater precifion ; even when 

 the work is performed by a careful defigner, who is no ma- 

 thematical inftrument maker. It is certain that much greater 

 delicacy and precifion may be had in the divifion of mathema- 

 tical inflruments by the patient diligence of a cultivator of 

 pradical mechanics than is generally fuppofed. 



ail. 



